The labile component of the depolarizing bipolar cell response to light was stabilized during whole-cell recording in salamander retinal slices by the addition of GTP to the patch electrode filling solution. This finding is consistent with a recent suggestion that the photoreceptor input is mediated by a G protein. The inward current associated with this input was still present at a low intracellular chloride concentration and may be assumed, therefore, to be carried by a cation influx.